With the battery capacity and average current consumption, you can compute the expected runtime of your project by solving the equation: Battery capacity (in mAh) / Average current consumption (in mA) = Hours of expected runtime For example if your project consumes 150 mA and your batteries are rated at 800 mAh at that current draw, you can can compute: 800 mAh / 150 mA = 5.33 hours expected runtime